I was in a charms shop a while back and they had a section laid out for Celtic Mythology charms and their meanings. I had a look and found Thor's hammer. I ask my mom and she said the Mythologies are connected. I ignore this with my small mind, thinking, "How?"

Then I get some money and took the family to Barns and Nobles. There I found a Norse Mythology book by Helen A. Guerber (I have yet to read it, but will when I get back from Basic Combat Training and AIT). I look at the cover and ask mom who it is. She looked and suggested the Green Man from Celtic Mythology.

Just geographically it's a reasonably quick journey from the Viking lands (Norway, Sweden, Denmark) to the Celtic/Pict ones (Scotland, England), so it's not surprising that there's overlap. [Also, why else would the adult Vikings in How to Train Your Dragon have Scottish burrs?]
– Lauren IpsumJul 10 '18 at 9:56

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@LaurenIpsum as well as the fact that Vikings invaded the Celtic lands (albeit by this time Celtic paganism was barely being practiced, if at all). It's understandable that they would pick up the old mythology of the places they invaded. I'm still looking for a more specific answer though.
– SamJul 28 '18 at 7:27

2 Answers
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How are Norse and Celtic mythology related? In a few different ways, I suppose.

Because they are the mythologies of similar human societies.

An example: the “Green Man” isn’t exclusively Celtic, faces made from or surrounded by leaves, fruits, vines or branches, or men with green skin, are found in ancient cultures throughout the world. Often they are related to dieties of rebirth and regeneration, responsible for the return of spring. In a society in the climate of north-western Europe during the first millennium, with rather “primitive” agriculture, without supermarkets and without refrigerators, people were much more dependent on the rebirth of nature in spring for their food supplies. A long winter could literally cost lives, so it is no wonder the return of spring was seen as a godly gift. With the earth turning green again, and plants getting leaves out of nowhere, it doesn’t seem such an illogical step to represent the god responsible for this as a green man surrounded by leaves.

Joseph Campbell, autor of a popular book with the title The Hero with a Thousand Faces, even goes as far as proposing a basic structure for myths in general, based upon a Jungian conception of archetypes.

Because the roots of Norse mythology supposedly lay in a Common Germanic mythology, which in turn goes back to a common (Western) Indo-European mythology from which also Celtic mythology is thought to originate.

An example: The celtic god Taranis’ name (meaning “thunderer”) is likely connected to Norse Thor (and Anglo-Saxon Thunor, German Donar, all from Common Germanic Thunraz meaning “thunderer”). Even the name of the Hittite god Tarhun might reflect the same root. But there is more: in Norse mythology, the mother of Thor is Fjörgynn. Her name is etymologically equivalent to the names of thunder gods in a lot of other Indo-European cultures, and even of other cultures influenced by the Indo-European: Slavic Perun, Lithuanian Perkunas, Sanskrit Parjanya, Albanian Perëndi, Thracian Perkon… The name is associated with the oak-tree. Maximus Tyrius in his 38th dissertation states that the Celts, too, venerate their Jupiter (i.e. their god of thunder) not with statues but by worshipping oaks instead. Germanic pagans are said to have worshipped Donar’s oak, according to Wilibald in his Life of Saint Boniface. Also the Greek God of thunder, Zeus, was worshipped in an oak tree at the ancient oracle of Dodona. Etc. etc.

Because they are mythologies that were in contact with each other during a considerable period of time, which favours mutual cultural influences.

As a matter of fact, Germanic and Celtic cultures have been in close contact during a long time. When the Norwegians started settling the Faroe islands and Iceland, they encountered Gaelic monks who were already living there. Lots of vikings also settled on the British isles and assimilated into society, but not without leaving traces in the culture. For example, Viking ships and weapons became models for later Irish ship and weapon designs.

Actually, the ancient people of northern Europe are all related (for all intents and purposes shall be referred to as Nords); as are those of the south (the pre-migration civilisation/culture known as the Pelasgians in Roman and Greek sources and were their predecessors).

One only need to look at and compare myths, language (particularly sound and core words) and art/material culture. It is technically inaccurate in most cases to claim a connection/ difference between Celtic and Norse/Germano-northern European myth considering the absence of written records during the time of a fully realised, living breathing Celtic pagan tradition before the mass conversion to Christianity and/or interpretatio Romana or the influence of either (this includes Ireland and Wales, from which current mythic texts derive).

Despite this, for the discerning scholar, even the surviving myths, folk tales and legends recorded in works such as the Cath Maige Tuired and the Lebor Gabála Érenn of Irish myth, and the Four Branches of the Mabinogi of the Welsh; all of which were written during the Middle Ages (prior to the Norse invasions) and after the Christian conversion, adapted from a muddled and compromised oral tradition stooped in taboo. Nonetheless, they hold much insightful information.

Note, the only (potentially) unaffected Celtic cult to survive for quite some time (some claim to this day, though that is not for me to argue) is that of the Druids, whom it is noted the Danes (Vikings) respected and saw much solidarity. To add to that, it should also be noted that a similar issue details much of 'Germanic', 'Slavic' and 'Uralic' myths and at times, even known gods; due to Christianisation ~ take the Prose Edda as a prime example!

All of this does not deny close contact and back and forth influence through time, as the days of the Nords and Pelasgians stretch far into prehistory, their true myths becoming subject to mutation as they became the latter tribes we are more familiar with: Pelasgians) Greeks, Italics (including Romans, Etruscans etc.), Thracians, Iberians, Anatolians, Illyrians. Nords) Celts, Scandinavians/Norse (N. Germanic), Germans (S. Germanic ~ Anglos, Saxons, Goths etc.), Uralics, Slavs ~ These are just to name those I can remember without inquiry, and is not limited to European peoples, if one observes carefully, the world is full of allusions to the once great "core" cultures of the most distant past (Indo-European is a fallacy, derived from inaccurate reconstruction based on a misunderstanding of lingua franca languages and the koine which may develop from them such as Sanskrit, as being purely original and therefore related via common ancestry).